Series led backlight control circuit

ABSTRACT

A light emitting diode control circuit provides a plurality of series light emitting diodes (LEDs) that are fault tolerant, temperature compensated, and temperature derated. The series LEDs may be used to backlight an LCD in such applications as laptop computers, personal digital assistants, cellular telephones and automotive applications. An optional luminance compensation circuit adjusts the current through the LEDs as a function of an LED temperature to maintain a substantially consistent LED intensity. An optional temperature derating circuit reduces the current through the LEDs when the temperature reaches a threshold. The LED temperature may also be provided externally via a temperature output signal.

REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/261760, filed Jan. 16, 2001, titled “AMLCD LEDBacklighting Navigation Radio Display” which is incorporated herein byreference.

BACKGROUND

[0002] This invention relates generally to the field of light emittingdiode (“LED”) circuits, and more specifically to the fields of controlcircuits for backlighting of LCDs and other devices, where the LEDcircuit is fault tolerant.

[0003] Backlighting for active matrix liquid crystal displays (“AMLCD”)typically uses a cold cathode fluorescent lamp (“CCFL”) device. CCFLdevices tend to have high back lighting efficacies. CCFL devices havenumerous drawbacks. For example, CCFL devices may contain Mercury, ahighly dangerous substance that has been banned from many AMLCDapplications. CCFL devices may have poor efficacy at lower temperatures,which requires additional circuitry such as a heater element or a boostcurrent circuit. CCFL devices may have a non-linear efficacy curve withrespect to temperature. CCFL devices may require an inverter to drivethe CCFL device. CCFL devices may require complex control schemes,including light sensors and temperature sensors to provide adequatedimming ratios for night time operations. CCFL devices may have a shortlife expectancy, especially at lower operating temperatures, and mayrequire additional electromagnetic interference (“EMI”) shielding andelectric filtering.

[0004] Alternatives to CCFL devices for back lighting an AMLCD includeXenon-based devices. Xenon-based backlighting circuits do not containMercury, have superior low temperature life expectancy and lowtemperature operational characteristics, and have less phosphordegradation than CCFL devices. However, Xenon lamps tend to berelatively expensive and require complex control circuitry. Xenon lampshave low efficacy. For example, a Xenon lamp with twice the diameter mayprovide only half the brightness of a mercury-based CCFL lamp. Becausethe efficacy of the Xenon lamp may be less than half of a CCFL lamp, theadditional power needed to power a Xenon based circuit creates a problemof power consumption. While Xenon lamps correct many of the problems ofthe CCFL lamp technology, the Xenon lamp technology creates many newproblems. Thus, there is a need in the LCD field to create a new anduseful back light device and drive circuit.

SUMMARY

[0005] A light emitting diode control circuit provides a plurality ofseries light emitting diodes (LEDs) that are fault tolerant, temperaturecompensated, and temperature derated. The series LEDs may be used tobacklight an LCD in such applications as laptop computers, personaldigital assistants, cellular telephones and automotive applications. Anoptional luminance compensation circuit adjusts the current through theLEDs as a function of an LED temperature to maintain a substantiallyconsistent LED intensity. An optional temperature derating circuitreduces the current through the LEDs when the temperature reaches athreshold. The LED temperature may also be provided externally via atemperature output signal.

[0006] The foregoing discussion has been provided only by way ofintroduction. Nothing in this section should be taken as a limitation onthe following claims, which define the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The components in the figures are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention. Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

[0008]FIG. 1 illustrates a block diagram of a series LED controlcircuit;

[0009]FIG. 2 illustrates a block diagram of a series LED controlcircuit;

[0010]FIG. 3 illustrates a circuit diagram of a series LED controlcircuit; and

[0011]FIG. 4, illustrate a flow diagram of a method of controlling aseries light emitting diode array.

DETAILED DESCRIPTION

[0012] A. Definitions

[0013] AMLOD—Active matrix Liquid Crystal Display.

[0014] CCFL—Cold cathode fluorescent lamp.

[0015] CCFL inverter—A circuit that provides the necessary voltage andcurrent to properly control the light output of a LCD CCFL light output.

[0016] LCD—Liquid crystal display.

[0017] Efficacy—The conversion efficiency of converting Watts intolumens (lumens/watt)

[0018] LED—Light emitting diode.

[0019] Lumen—A unit of light power useful to the human eye and definedas the spectral luminous efficacy for monochromatic light at the peakvisual response wavelength of 555 nm.

[0020] NIT—A unit of luminance for light reflected, transmitted, oremitted by a diffusing surface.

[0021] PWM—Pulse width modulation.

[0022] B. Introduction

[0023] The improved LED circuit provides backlighting for AMLCDs withnumerous LEDs, eliminating many of the problems associated with CCFL andXenon systems. The improved LED circuit also provides improved LEDcontrol for other applications. The LEDs are configured in series with adiode or similar device in parallel to provide fault tolerance. Acurrent sampling circuit detects the current flowing through the LEDsand adjusts the LED input voltage accordingly.

[0024] An optional temperature compensation circuit may be used tomaintain the desired luminance of the LEDs as a function of thetemperature of the LEDs. An optional temperature derating circuit may beused to reduce the LED current when the LED temperature exceeds atemperature threshold. The temperature derating circuit may increase thelife expectancy of the LEDs.

[0025] Such an LED circuit may be used in systems for backlighting LCDs.Such systems may be used in automotive applications, computerapplications including laptop computers, personal digital assistances,cellular telephones, and others. A backlighting system with multipleLEDs connected in a series configuration instead of in a parallelconfiguration has less power dissipation. Such system may haveapproximately 25% less power dissipation. This is especially importantfor battery operated devices, such as laptops PDA, cellular telephone,etc., and devices with limited heat dissipation capabilities.

[0026] The following description of the invention is not intended tolimit the scope of the invention, but rather to enable any personskilled in the art to make and use the invention.

[0027] C. The Light Emitting Diode Circuit

[0028]FIG. 1 illustrates a fault tolerant series LED system 100. The LEDsystem 100 may include an array of LED connected in series and parallelelements connected in parallel with the LEDs 110, a current monitor 120,a voltage converter 108, and an optional temperature derating andbrightness circuits. The optional temperature derating and brightnesscircuits may include a temperature sensor 122, an amplifier 124, adisplay luminance microprocessor 102, and a temperature derating circuit106. The LED system 100 may include only some of these components, othercomponents, and/or equivalent components.

[0029] The LED array 110 may include a plurality of LEDs 116 and 118connected in series. The LEDs may include white or colored LEDS, such asred, green, blue, or another colored LEDS, or a combination of differenttypes of LEDs. While FIG. 1 illustrates only two LEDs in the LED array110, the LED array 110 may have any number of LEDs, for example 2 to50,000 LEDs or more. The parallel elements 112 and 114 provide faulttolerance for open circuits in the series LEDs. An open circuit may becaused by a failed LED, a failed solder connection, or other failure.The parallel elements 112 and 114 are each connected in parallel withone or more LEDs to provide fault tolerance. Some sapphire-based LEDhave zener diodes included in the package for electrostatic dischargeprotection purposes. In a first embodiment, each parallel element 112and 114, respectively is connected in parallel with a single LED 116 and118, respectively, as illustrated in FIG. 1. In a second embodiment,some or all of the parallel elements 112 and 114 are connected inparallel with more than one LED. For example, each parallel elementcould be connected in parallel with five LEDs, such that if any of thefive LEDs fail by an open circuit, the parallel element will route thecurrent around all five LEDs. This allows for fewer parallel elements,however, such parallel element would need to have a higher voltage drop,approximately five times larger than the parallel elements in the firstembodiment. In other examples of the second embodiment, the parallelelement may be in parallel with two, three, four or more LEDs. Theparallel element may be a zener diode or other circuit. A parallelelement and a LED may be packaged together as a single unit. Forexample, some sapphire-based LEDs have zener diodes included in thepackage for electrostatic discharge protection purposes. The LED array110 could include a plurality of such packages connected in series.

[0030] The LED system 100 may operate with various supply voltages, forexample 1 volt to 45 volts. The LED system 100 may operate at automotivepower levels, for example, approximately 12-14 volts. The LED system 100may be a band limited low electromagnetic interference circuit.

[0031] When an LED 116 or 118 fails, the parallel element 112 or 114connected in parallel with the LED will be activated and route thecurrent flows around the failed LED, thus allowing the remaining LED tooperate properly. The failure may be caused by an LED failure, a solderjoint failure or other type failure. The parallel elements 112 and 114may be in an inoperative state until an LED fails. Thus, the parallelelements 112 and 114 would consume little or no power until an LEDfailed. Once an LED fails, the parallel element 112 or 114 would beactivated.

[0032] The parallel elements 112 and 114 may comprise a circuit such asa zener diode or other device. The parallel elements 112 and 114 may beconnected is series with each other or may simply be connected inparallel with one or more LEDs. The parallel elements 112 and 114 may beany circuit or circuits that pass little or no current at the LEDs' 116and 118 normal operating voltage range and pass current at voltageslarger than the LEDs' 116 and 118 normal operating voltage range. Forexample, parallel elements 112 and 114 may be a zener diode with aturn-on voltage larger than the LEDs' 116 and 118 normal operatingvoltage range. Alternatively, the parallel elements 112 and 114 may be atransistor biased with two resistors to turn on the base-emitterjunction when the voltage from the collector-to-base exceeds the LEDs'116 and 118 normal operating voltage range.

[0033] The voltage converter 108 may be a high voltage converter orother device that provides an adjustable output DC voltage. The voltageconverter 108 adjusts the output voltage such that the current throughthe LEDs 116 and 118 remains substantially constant in proportion to thecommanded current signal. That is, the LED current level will changewhen the display luminance is changed. The voltage converter 108generates an output voltage as a function of a supply voltage 104, acurrent flow signal from the current monitor 120, and/or a commanded LEDcurrent signal from the display luminance microprocessor 102 as modifiedby an optional temperature derating circuit 106.

[0034] The microprocessor 102 determines the appropriate LED currentthat corresponds to the user's commanded luminance. The microprocessor102 modifies the LED current as a function of the LED temperature toaccount for the LED efficacy changes as the LED temperature changes. Thetemperature derating circuit 106 may optionally derate the LED currentif the LED temperature becomes too hot. In a system 100 with both atemperature compensation circuit, e.g. the microprocessor 102, and atemperature derating circuit 106, the temperature derating circuit 106may override the commanded current signal from the microprocessor 102when the LED temperature becomes too hot.

[0035] The voltage converter 108 may include a pulse width modulationconverter controller IC, a digital signal processor (DSP), or other typeof circuit. The voltage converter 108 provides a current to the LEDs todrive the LEDs at a commanded current generated by the microprocessor102. The voltage converter 108 may use a current flow signal, alsocalled a current feedback signal, from the current monitor 120 tomaintain the commanded current level. When the LED array 110 has an opencircuit due to an LED failure or an LED solder joint failure, thevoltage converter 108 detects that no current is passing through thecircuit and will increase the voltage, possibly creating an over-voltagecondition. Thus, the parallel elements 112 and 114 help to prevent suchan over-voltage condition in the voltage converter 108 by providing abypass path.

[0036] The current monitor 120, also called a current sampling circuitor an LED current sample resistor, provides a current flow signal to thevoltage converter 108. The voltage converter 108 uses this signal todetermine the optimal voltage to supply the LEDs to maintain the desiredLED current. Since the optimal LED voltage varies as function oftemperature and from device to device, the luminance needs to be sampledto determine the LED's current, which is proportional to the LEDluminance. Generally, as an LED heats up the LED generates lessluminance for a fix current flow through the LED. An LED tends togenerate more luminance when more current flows through the LED. Thecurrent monitor 120 may be a resistor connected between the series LEDsand ground. The signal from the current sampling circuit may be thevoltage drop across the resistor.

[0037] When an LED 116 fails that causes an open circuit in the LEDseries circuit, either by an LED burn-out or an open circuit, or asolder joint failure, the current flow signal from the current monitor120 indicates that an open circuit has occurred. The voltage converter108 then increases its output voltage in an attempt to maintain thedesired current through the LEDs 116 and 118. As the voltage converter'soutput voltage is raised, the parallel element 112 that is connected inparallel with the failure will turn on and providing a current bypass ofthe failure. By bypassing the failure, the remaining LED 118 continuesto operate correctly. While an LED failure by a short is an unlikelyfailure, such a failure will not prevent the other LED from remaining onat the desired luminance. The current monitor 120 will compensate forthe reduced load when an LED shorts out. Thus, the brightness of theother LED is unaffected by the shorted or opened LED.

[0038] Because LED's luminance or efficacy changes as a function oftemperature, a temperature compensation system is desirable to maintainthe desired luminance across a range of temperatures. Since therelationship between an LED's luminance and temperature is notnecessarily linear, a display luminance microprocessor 102, also calleda temperature compensation circuit, may be used to more accuratelycontrol the LED's luminance. Maintaining consistent LED luminance isespecially important for night time applications, such as inautomobiles, laptop computers, personal digital assistances, andcellular telephones. Also, as an LED's temperature increases, the LED'slife expectancy may decrease.

[0039] The optional temperature derating system may include atemperature sensor 122, an amplifier 124, and/or a temperature deratingcircuit 106. Alternatively, the derating function may be accomplished bythe microprocessor 102. When the LED temperature exceeds a temperaturethreshold, the temperature derating circuit 106 reduces the currentflowing through the LEDs to prevent LED burnout.

[0040] The temperature sensor 122, also called an LED temperaturesensing device or a temperature resistor or a temperature monitoringcircuit, may be a temperature sensitive device thermally and/orelectrically connected with a cathode terminal of an LED. Thetemperature sensor 122 may measure the solder temperature near thecathode terminal. The LED's temperature may be inferred from the soldertemperature. This temperature may be used to compensate and/or deratethe LED array. The solder temperature may be converted into an ambienttemperature, which can be used to derate the LED from the LED'sspecification sheets. The temperature signal may be sent to theamplifier 124 then to the display luminance microprocessor 102 and thetemperature derating circuit 106. The display luminance microprocessor102 sends a “commanded current” signal to the temperature deratingcircuit 106 based on the amplified temperature signal.

[0041]FIG. 2 illustrates a block diagram of a series LED control circuit200. The LED control circuit 200 adjusts the current provided to theLEDs 210 based on an LED temperature. As an LED heats up, the brightnessdecreases at the same current. Thus, to maintain consistent brightnessas an LED heats up, the LED's current must be increased. Since therelationship between the temperature and the LED's brightness is notlinear, a software solution may be desirable.

[0042] A command brightness signal 202 that indicates the desiredbrightness of the LED 210 is received by the microprocessor 204. Amicroprocessor 204 then uses a digital temperature signal from the A/Dconverter 216 to determine the appropriate temperature correction factorfrom the temperature correction factor table 206. The microprocessor 204sends an adjusted brightness signal to the series LED drive circuit 208.The adjusted brightness signal may be a PWM signal. The temperature ofthe LED 210 is measured by a thermal resistor 212, labeled “Rt.” Thetemperature compensation and control circuit 214 receives thetemperature measurement from the thermally sensitive resistor 212. Thetemperature signal is then converted in a digital signal by an A/Dconverter 216 and sent to the microprocessor 204.

[0043]FIG. 3 illustrates a circuit diagram of an embodiment of a seriesLED system. The LED system 300 may include an LED array 306, a voltageconverter 304, a brightness control circuit 308, a temperature deratingcircuit 310, and a temperature monitoring circuit 312.

[0044] The brightness control circuit 308 receives a brightness signalat node 302 and controls the brightness of the LED array 306. Thebrightness signal may be a desired brightness level for the LED array306, for example daytime brightness or night time brightness in anautomotive radio display. The operational amplifier U2 is configured asa differential amplifier where the ratios of the operational amplifier'sresistors are substantially balanced. That is, R12/R11=R10/R9. When theratios of the operational amplifier's resistor R12/R11 and R10/R9 areboth substantially equal to one, the differential gain of theoperational amplifier U2 is substantially unity.

[0045] The input node 302 of the LED circuit 300 may receive an inputsignal from a microprocessor or other controller. The input signal maybe a pulse width modulated (“PWM”) signal, a DC voltage signal, or othertype of signal. A PWM input signal controls the intensity of the LEDbased on the duty cycle and/or the voltage level of the input signal.Generally, as the duty cycle of the input signal increases, the LEDsbecome brighter. A DC voltage input signal controls the intensity of theLED based the voltage level of the input signal. Generally, as thevoltage level at the input node 302 increases, the LEDs become brighter.

[0046] The temperature derating circuit 310 derates the current suppliedto the LED array 306 when the LED temperature increases beyond athreshold. Derating the LED array 306 current prolongs the lifeexpectancy of the LEDs. The output of the operational amplifier U4 is atsubstantially ground when no temperature derating is required and theoperational amplifier U2 passes the brightness signal from input node302 with the gain set by the ratios of the resistors R9-R12, which maybe a unity gain. The brightness signal may be a steady DC voltage, apulse width modulated signal, or an other type of signal.

[0047] The derating operational amplifier U4 normally operates in arail-to-rail mode. When the LED array 306 is operating in a normaloperating temperature range, the output of the derating operationalamplifier U4, known as the temperature derating level, is substantiallyground. As the temperature of the LED array 306 increases, thetemperature derating level increases after a predetermined LED thresholdtemperature is reached. Since the thermal resistor RT1 is thermallyconnected with the same thermal plane as the LED array 306 is heat sunkto, the resistance of the thermal resistor RT1 varies as a function ofthe temperature of the solder near the cathode terminals of the LEDs.The thermal resistor RT1, also called a temperature sensor, has aresistance that varies as a function of a measured temperature. Forexample, the thermal resistor RT1 may be a model KT230 available fromInfineon Technologies A.G. The model KT230 is a temperature dependentresistor with a resistance tolerances of ±3% at 1,000 Ohms, atemperature range of −50 degree Centigrade to +150 degree Centigrade,and is available in SMD or leaded or customized packages. The modelKT230 has a linear output, a positive temperature coefficient, a longterm stability, a fast response time, and is polarity independent due tosymmetrical construction. Other commonly available temperature sensors,such as models LM135 and LM50 from the National Semiconductor, Inc., mayalso be used.

[0048] When the operational amplifier U2 receives an output voltagegreater than zero volts from the derating operational amplifier U4through resistor R11, the output voltage acts as a negative offset tothe input voltage at the input node 302. The LEDs then become lessbright as the temperature increases. For example, if the brightnesssignal at the input node 302 is 5 VDC and the temperature derating levelis 1.5 V, the output of the operation amplifier U2 is substantially 3.5V. The temperature derating circuit 310 may shut off the LED array 306if the measured temperature reaches a predetermined temperaturethreshold.

[0049] The temperature monitoring circuit 312 provides a temperatureoutput signal at output node 310 that indicates a temperature associatedwith the LED array 306. The LED temperature output signal may be afunction of the LED temperature as measured by the thermal resistor RT1.The thermal resistor RT1 may be used for the temperature monitoringcircuit 312 and the temperature derating circuit 310. The temperaturemonitor amplifier U3 monitors a voltage difference between a firstvoltage divider circuit R19 and R20 and a second voltage divider circuitR17 and RT1. The output of the temperature monitor amplifier U3 isconnected with the output node 310.

[0050] D. Example Embodiment of the LED Circuit

[0051] The LED system 300 of FIG. 3 may include components as indicatedin Table 1. Other types of components and components of different valuesmay also be used in the LED system 300 as will be apparent to one ofskill in the art. TABLE 1 Ref. Description LED1-4 Light emitting diodes(LEDs). For example, a white LED from Infineon model LW E673 or LW E67C,red LED model LSA677-Q, green LED model LTA673-R24, or a blue LEDLBA673-N24 all from Infineon Technology AG. U2-4 Operational amplifiers,for example a model LMV321 available from National Semiconductor Corp.or a model TLC 2274 Rail-to-Rail Operational Amplifier available fromTexas Instruments, Inc. VR1-4 Zener diodes, for example a 5.1 V zenerdiode. R9-21 Resistors, for example a 20 K Ohms resistor. Otherresistance values may also be used, for example, 200 to 200 K Ohms. RTIA resistor with a temperature dependant resistance, for example KT230available from Infineon Technology A.G. Rs A resistor, for example a 165Ohm resistor.

[0052] E. Method Of Controlling LEDs

[0053] Referring to FIG. 4, a method 400 of controlling a series lightemitting diode array is illustrated. In block 402, the temperature ofthe light emitting diode array is monitored. In block 404, the LEDs aretemperature compensated. The current to the light emitting diode arrayis adjusted as a function of the monitored temperature to maintainconsistent LED luminance. Because as an LED's temperature increases theLED's luminance decreases, as the LED's temperature increase, thecurrent through the LED needs to be increased to maintain consistentluminance.

[0054] In block 406, the LEDs are temperature derated. If the LEDtemperature exceeds a temperature threshold, for example, 55 degreesCelsius, the input current to the LED array is reduced. The temperaturethreshold may vary depending on the type of LED, the application, andother considerations. Temperature derating reduces the likelihood of LEDburn out and may be used only when the LED is near the brink of a burnout. The method 400 may be run constantly or may be run periodically.Portions of the method 400 may be performed with hardware, software, ora combination of hardware and software.

[0055] F. Application of the LED Backlighting System

[0056] The LED devices and systems describe above may be used in avariety of systems including an LCD backlighting display unit adaptedfor an automotive application. Such a backlighting display unit mayinclude a liquid crystal display, a plurality of light emitting diodesconnected in a series configuration, and a plurality of parallelelements connected in parallel with the light emitting diodes such thatcurrent is routed around a light emitting diode with a failure when thefailure comprises an open circuit.

[0057] As a person skilled in the art will recognize from the previousdescription and from the figures and claims, modifications and changescan be made to the preferred embodiments of the invention withoutdeparting from the scope of the invention defined in the followingclaims.

What is claimed is:
 1. A light emitting diode device, comprising: aplurality of light emitting diodes connected together in series; aplurality of parallel elements connected in parallel with the pluralityof light emitting diodes; a current monitor connected with the pluralityof light emitting diodes that measures an amount of current flowing fromthe plurality of light emitting diodes and generates a current flowsignal; and a voltage converter that supplies a current to the pluralityof light emitting diodes as a function of the current flow signal and acommanded current signal.
 2. The device of claim 1, wherein thecommanded current signal comprises a direct current signal.
 3. Thedevice of claim 1, wherein the commanded current signal comprises apulse width modulated signal.
 4. The device of claim 3, wherein thecommanded current signal is generated by a microprocessor.
 5. The deviceof claim 1, wherein the plurality of parallel elements comprises aplurality of zener diodes.
 6. The device of claim 1, wherein a parallelelement is connected in parallel with a light emitting diode.
 7. Thedevice of claim 1, wherein a parallel element is connected in parallelwith multiple light emitting diodes.
 8. The device of claim 1, furthercomprising: a temperature sensor that measures a temperature associatedwith at least one of the plurality of light emitting diodes andgenerates a temperature signal.
 9. The device of claim 8, furthercomprising: a temperature derating circuit that reduces the current tothe plurality of light emitting diodes the temperature signal exceeds atemperature threshold.
 10. The device of claim 9, wherein thetemperature derating circuit adjusts the commanded current signal suchthat the voltage converter supplies less current to the plurality oflight emitting diodes.
 11. The device of claim 9, wherein thetemperature sensor measures a solder temperature near a light emittingdiode.
 12. The device of claim 11, wherein the linear temperature sensorcomprises a temperature dependant resistor.
 13. The device of claim 12,wherein a terminal of the temperature dependant resistor and a cathodeterminal of a light emitting diode are thermally interconnected.
 14. Thedevice of claim 9, wherein the temperature derating circuit comprises amicroprocessor.
 15. The device of claim 14, wherein the temperaturederating circuit provides a signal to the voltage converter as afunction of a measured temperature and a temperature correction factortable.
 16. The device of claim 8, further comprising: a temperaturecompensation circuit that adjusts the current to the plurality of lightemitting diodes as a function of the measured temperature.
 17. Thedevice of claim 16, the temperature compensation circuit adjusts thecurrent to the plurality of light emitting diodes such that theplurality of light emitting diodes have a substantially consistentluminous intensity when the measured temperature increases.
 18. Thedevice of claim 1, wherein the light emitting diode control device is aband limited low electromagnetic interference circuit.
 19. The device ofclaim 1, wherein the plurality of parallel elements being connected inparallel with the plurality of light emitting diodes such that currentis routed around a light emitting diode with a failure, where thefailure is an open circuit.
 20. The device of claim 1, wherein theplurality of light emitting diodes are adapted to provide back lightingfor an active matrix liquid crystal display.
 21. A display unit adaptedfor an automotive application, comprising: a liquid crystal display and;a backlighting array comprising a plurality of light emitting diodes ina series configuration and a plurality of parallel elements connected inparallel with the light emitting diodes such that current is routedaround a light emitting diode with a failure when the failure comprisesan open circuit.
 22. The display unit of claim 21, further comprising: atemperature derating circuit electrically connected with thebacklighting array, wherein the temperature derating circuit measures alight emitting diode temperature and reduces a current supplied to thebacklighting array if the light emitting diode temperature exceeds athreshold.
 23. The display unit of claim 22, further comprising: atemperature compensation circuit electrically connected with thebacklighting array, wherein the temperature compensation circuitmeasures a light emitting diode temperature and adjusts the currentsupplied to the backlighting array as a function of the light emittingdiode temperature such that the plurality of light emitting diodes havea substantially consistent luminous intensity when the light emittingdiode temperature increases.
 24. The display unit of claim 23, furthercomprising: a microprocessor-based light emitting diode controller thatprovides a pulse width modulated signal that controls the intensity ofthe light emitting diode array.
 25. A method of controlling a serieslight emitting diode array, comprising: monitoring a temperature of thelight emitting diode array at a node connected with a light emittingdiode; and adjusting an input current to the light emitting diode arrayas a function of the temperature.
 26. The method of claim 27, furthercomprising: monitoring a current from the light emitting diode array;and adjusting the input voltage as a function of the current.